The measurement and control of light levels in the built environment is important for many reasons. In respect of safety, areas need to be well-enough lit to allow users of the environment to move around safely, and that sufficient light must be provided such that hazards may be readily seen. Suitable light levels are also required for the comfort and convenience of users, who might e.g. be in a situation where they wish to read, or transact business. From an environmental and commercial viewpoint, it is also important that artificial light is not used when it is not necessary. This saves money, and lessens the impact of energy use on the environment.
Most built environments obtain their light through a combination of daylight and artificial light, with the balance of one over the other being dependent on the local light levels. As the seasons pass, the time at which it becomes necessary to switch on artificial lighting varies. As a result, it is not easy to design timer-based systems that switch lights on and off at the appropriate time, in order to maintain a desired illumination intensity. Furthermore, the light intensity produced at various locations within any particular built environment will depend not just on the strength of the daylight at any one time, but also the physical shadowing of region by the structures present in the environment.
The issues can be exemplified by considering a built environment such as a railway station. The station might comprise features such as a passenger car park, a covered pedestrian region in front of the station, a booking hall (that might have windows to the outside), a station concourse, railway platforms (usually with a some kind of overhead weather protection), waiting rooms, passenger bridges to cross over railway lines and stairs leading from the bridges to each platform. It is quite possible that some regions (e.g. the concourse, booking hall and passenger bridges) might have not only windows, but translucent roof panels, to admit natural light into the area.
Each of these areas will also have a different requirement for minimum light intensities. For example, a car park region would normally be expected to be less bright than a station concourse. A booking hall, where passengers might be transacting business and checking timetables, is likely to need higher illumination than the concourse itself. Adequate light levels will be needed on passenger bridges, but required light levels on stairs leading to and from the bridges are likely to be higher. In some situations, minimum light intensities are set by regulation, such as the light levels required within a set distance from a platform edge.
In view of all these requirements, lighting systems are controlled in a number of ways. A timer-based system may be used to switch artificial lights on and off at pre-set times. This causes difficulty as the hours of darkness vary through the seasons, and different timers would be needed for each region in the environment. A commonly used system to overcome these problems is to install light sensors in important regions that sense the intensity of local ambient light, and control the operation of local lighting. This is usually accomplished by the use of a set-point on the sensor, such that the light is switched on when light intensity drops below a desired value, and switched off when the ambient light intensity rises above a desired value. A degree of hysteresis is usually included in the controllers to prevent the control system switching lights on and off when light levels hover around the set-point, and the light sensors are usually positioned such that they are not illuminated by the light itself. This is the system that is used commonly on urban or suburban street lights, where each luminaire is provided with an upwardly-facing light sensor. The provision and maintenance of these multiple sensors is costly, and identification of an appropriate set point is often either not easy, or is difficult to change once the sensor has been installed.
It is among the objects of the present invention to attempt a solution to these and other problems.